Limit switches are widely used in various industrial applications, and they can detect a limit of movement of an article and passage of an article by displacement of an actuating part such as a pivotally supported arm or a linear plunger. For proper operation of such limit switches, it is necessary for the actuating part to undergo a certain displacement. However, even when a limit switch is initially mounted at an appropriate location, its position may shift in time. If such a shift is sufficiently small, no appreciable consequence may develop. However, it means that the safety margin of the operation of the limit switch may be substantially reduced without being known, and it may lead to an abrupt failure of the limit switch.
To avoid such an abrupt failure, it is desirable to monitor the safety margin of the operation of the limit switch so that an early warning may be made in case a potentially unreliable state of the limit switch is produced.
However, it is quite time consuming and difficult to inspect the accuracy of the position of each of a large number of limit switches that are used in various parts of a large manufacturing systems where such limit switches are typically used. Further, during the operation of such a system, most of the limit switches are typically inaccessible due to the presence of articles which are being conveyed and a considerable difficulty is encountered when an attempt is made to inspect the accuracy of the mounting positions of limit switches.